Mission Design

A direct airplane flight might be the quickest way across the country, but the fastest route to Pluto requires a trip past Jupiter. The giant planet's gravity can actually "slingshot" a spacecraft toward the outer solar system.

There are two reasons why the New Horizons science team wants to reach Pluto and Charon as soon as possible. The first has to do with Pluto's atmosphere: Since 1989, Pluto has been moving farther from the Sun, getting less heat every year. As Pluto gets colder scientists expect its atmosphere will "freeze out," so the team wants to arrive while there is a chance to study a thicker atmosphere.

The second reason is to map as much of Pluto and Charon as possible. On Earth, the North Pole and other areas above the Arctic Circle have half a year of night and half a year of daylight. In the same way, parts of Pluto or Charon never see the Sun for decades at a time. The longer we wait, the more of Pluto and Charon are shadowed in a long "arctic night," impeding the spacecraft's ability to take pictures of the entire surface in reflected sunlight.

Prime Opportunity

By launching in January 2006, New Horizons could take advantage of a gravity assist from Jupiter. In February 2007, New Horizons passed through the Jupiter system at more than 50,000 mph, ending up on a path that gets it to Pluto on July 14, 2015.

During the cruise from Jupiter to Pluto, the mission team is monitoring the health of the spacecraft while planning and practicing for the encounter with Pluto and Charon. At the same time, observers used telescopes from Earth to find and study Kuiper Belt Objects that the spacecraft could fly by after Pluto (as part of a possible extended mission). The "KBOs" are ancient, icy bodies that orbit beyond Neptune.

The cameras on New Horizons will take data on Pluto and its moons months before the spacecraft arrives. Pluto and Charon will first appear as unresolved bright dots, but the planet and its moons appear larger as the encounter date approaches. Three months from the closest approach - when Pluto is about 65 million miles (105 million kilometers) away - the cameras on the spacecraft can make the first maps. For those three months, the mission team will take pictures and spectra measurements.

Pluto and Charon each rotate once every 6.4 Earth days. For the last two Pluto days before encounter (11 to 12 Earth days), the team will compile maps and gather spectra measurements of Pluto and Charon every half-day. The team can then compare these maps to check changes over a Pluto day, at a scale of about 30 miles (48 kilometers), as might be caused by new snow or other weather.

The Encounter

At closest approach, the spacecraft comes about 7,750 miles (12,500 kilometers) from Pluto and about 17,900 miles (28,800 kilometers) from Charon. On the way in, the spacecraft will look for ultraviolet emissions from Pluto's atmosphere and make the best global maps of Pluto and Charon in green, blue, red, and a special wavelength that is sensitive to methane frost on the surface. It will also take spectral maps in the near-infrared, telling the science team about Pluto's and Charon's surface compositions and locations and temperatures of these materials.

During the half-hour when the spacecraft is closest to Pluto or its largest moon, it will take close-up pictures in both visible and near-infrared wavelengths. The best pictures of Pluto will depict surface features as small as 200 feet (about 60 meters) across.

Even after the spacecraft passes Pluto and its moons, its work is far from done. Looking back at the mostly dark night-time side of Pluto or Charon is the best way to spot haze in the atmosphere, to look for rings, and to figure out whether their surfaces are smooth or rough. Also, the spacecraft will fly through the shadows cast by Pluto and Charon. It can look back at the Sun and Earth, and watch the light from the Sun or the radio waves from transmitters on Earth. The best time to measure the atmosphere happens as the spacecraft watches the Sun and Earth set behind Pluto and Charon.

Beyond Pluto

After passing through the Pluto system, pending NASA approval of an extended mission, the spacecraft can retarget itself for an encounter with a KBO. The KBO target will not be selected until later in the mission, but scientists are already studying several targets the spacecraft could possibly reach that are 15-30 miles (about 25-50 kilometers) across. This encounter would be similar to the Pluto encounter; the spacecraft would map the KBO, get its composition from infrared spectroscopy and four-color maps, and look for an atmosphere and moons.